Abacavir, is the International Nonproprietary Name (INN) of {(1S,4R)-4-[2-amino-6-(cyclopropylamino)-9H-purin-9-yl]-cyclopent-2-enyl}methanol, and CAS No. 136470-78-5. Abacavir sulfate is a potent selective inhibitor of HIV-1 and HIV-2, and can be used in the treatment of human immunodeficiency virus (HIV) infection.
The structure of abacavir hemisulfate salt corresponds to formula (I):

EP 434450-A discloses certain 9-substituted-2-aminopurines including abacavir and its salts, methods for their preparation, and pharmaceutical compositions using these compounds.
Different preparation processes of abacavir are known in the art. In some of them abacavir is obtained starting from an appropriate pyrimidine compound, coupling it with a sugar analogue residue, followed by a cyclisation to form the imidazole ring and a final introduction of the cyclopropylamino group at the 6 position of the purine ring. Pyrimidine compounds which have been identified as being useful as intermediates of said preparation processes include N-2-acylated abacavir intermediates such as N-{6-(cyclopropylamino)-9-[(1R,4S)-4-(hydroxymethyl)cyclopent-2-enyl]-9H-purin-2-yl}acetamide or N-{6-(cyclopropylamino)-9-[(1R,4S)-4-(hydroxymethyl)cyclopent-2-enyl]-9H-purin-2-yl}isobutyramide. The removal of the amino protective group of these compounds using acidic conditions is known in the art. According to Example 28 of EP 434450-A, the amino protective group of the N-{6-(cyclopropylamino)-9-[(1R,4S)-4-(hydroxymethyl)cyclopent-2-enyl]-9H-purin-2-yl}isobutyramide is removed by stirring with 1N hydrochloric acid for 2 days at room temperature. The abacavir base, after adjusting the pH to 7.0 and evaporation of the solvent, is finally isolated by trituration and chromatography. Then, it is transformed by reaction with an acid to the corresponding salt of abacavir. The main disadvantages of this method are: (i) the use of a strongly corrosive mineral acid to remove the amino protective group; (ii) the need of a high dilution rate; (iii) a long reaction time to complete the reaction; (iv) the need of isolating the free abacavir; and (v) a complicated chromatographic purification process.
Thus, despite the teaching of this prior art document, the research of new deprotection processes of a N-acylated {(1S,4R)-4-[2-amino-6-(cyclopropylamino)-9H-purin-9-yl]-cyclopent-2-enyl}methanol is still an active field, since the industrial exploitation of the known process is difficult, as it has pointed out above. Thus, the provision of a new process for the removal of the amino protective group of a N-acylated {(1S,4R)-4-[2-amino-6-(cyclopropylamino)-9H-purin-9-yl]-cyclopent-2-enyl}methanol is desirable.